The invention relates to a valve assembly for pressure adapted and volumetric flow adapted supply of at least one user in.
The like valve assemblies are employed, for example, in mobile hydraulics for controlling users, in particular single and double-action cylinders. Double-action cylinders are frequently employed in front power lifts of farming tractors. The previous rear power lifts were in most cases constructed with single-action cylinders, however owing to the variegated applications of modern tractors there is an increasing trend to also equip the rear power lifts with double-action cylinders. With the aid of such power lifts, various peripheral tools such as, for example, balers, ploughs, cultivators, rollers, etc. may be coupled to a tractor and actuated.
In mobile hydraulics it is an objective to make the design of the valve assemblies as compact as possible, for which reason they are frequently designed in subplate mounting or as a compact block or monoblock. To this end, the required ports such as, for example, pump port, control port, work port, tank port, and the housing bores necessary for receiving the valve actuating members are formed in the basic body of the valve plate or of the compact block.
In manually operated valve assemblies, the mechanical spool is designed to project from the valve housing (plate, block), and this external chamber of the valve axis is connected to the tank. In order to satisfy the modular principle, this construction intended for mechanically actuated valve assemblies is also applied to electrically actuated valves, so that considerable expense in terms of device technology must be incurred for realising the external pressure chambers and the corresponding connection conduits toward the tank.
In mobile hydraulics, load sensing systems are being employed whereby a through flow independent of load pressure, and thus sensitive velocity control of the user, is attained. Herein the pressure difference across the directional control valve is maintained constant by providing in the user ports individual pressure compensators which throttle the system pressure, i.e., the pressure of the highest load in the system, to the respective user pressure.
In the like load sensing systems, the individual pressure compensators and their control conduits must, accordingly, also be accommodated in the valve housing (plate, block) of the valve assembly.
DE 36 34 728 C2 discloses a valve assembly for load independent control of several double-action hydraulic users, wherein the metering orifice is realised in the form of fine control grooves of the directional control valve, and downstream from this metering orifice an individual pressure compensator is received in a valve housing bore through which the hydraulic fluid may be supplied to a first or second work port depending on actuation of the directional control valve spool. The individual pressure compensator comprises a piston acted upon in the opening direction by the pressure downstream from the metering orifice, and in the closing direction by a spring and a control pressure.
It is a drawback of this embodiment that a receiving bore for the piston of the individual pressure compensator and corresponding conduit systems for supplying the control pressure to the piston rear side must be formed in the housing of the valve assembly, so that considerable expense in terms of production technology is required for manufacturing the valve housing. It is another drawback that, upon use of a different individual pressure compensator, it may eventually become necessary to modify the valve housing bore, so that it is necessary to provide various valve housing construction types.
DE-OS 36 05 312 discloses a valve assembly wherein the directional control valve spool is designed as a hollow spool wherein pocket hole bores for receiving a respective piston of an individual pressure compensator from both end portions are provided. The metering orifice of the directional control valves is constituted by a jacket bore of the directional control valve spool and an annular chamber of the valve housing connected to the pump port. Through this jacket bore the hydraulic fluid may enter into one of the pocket hole bores in accordance with actuation of the directional control valve spool, so that the corresponding piston of the individual pressure compensator is displaced against a spring bias, and the corresponding work port is controlled open in order to supply the user, in this case a double-action hydraulic cylinder, with hydraulic fluid.
In this variant, a separate valve housing bore for receiving the individual pressure compensators need not be realised in contrast with the above described construction. The variant known from DE-OS 3 605 312 does, however, have the drawback that to each work port a separate individual pressure compensator is associated, resulting in a very complicated construction of the hollow piston. In addition, in such a construction it is necessary to select the tolerances in manufacture of the two individual pressure compensators very narrowly in order to attain identical response characteristics in both work ports. Any variations in the response characteristics might engender instabilities in actuation of the user, which certainly are not acceptable in view of the quality criteria to be applied nowadays.
An object of the present invention is furnishing a valve assembly for pressure adapted and volumetric flow adapted supply, whereby secure control of a user is ensured at minimum expense in terms of device technology.
This object is attained by a valve assembly having a continuously adjustable directional control valve associated with a pressure compensator having a piston, which is guided in a hollow spool of the directional control valve. The piston is acted upon in the opening direction by pressure downstream from a metering orifice constituted by the directional control valve, and is acted upon in the closing direction by a control spring and a control pressure. Through axial displacement of the piston, a jacket bore of said spool may be controlled open in such a way that a connection with one of two work ports may be selectively established depending on the spool position.
By the measure of guiding the piston of the individual pressure compensator in a hollow spool provided with a jacket bore, which may be controlled open by axial displacement of the pressure compensator piston on the one hand and, depending on the position of the directional control valve spool (hollow spool), establishes a connection with a first or second work port on the other hand, expense in terms of device technology may be reduced quite considerably in comparison with the above described solutions because neither a separate receiving bore for the individual pressure compensator in the valve housing nor corresponding means for receiving a second pressure compensator piston in the directional control valve spool must be provided.
The invention thus makes it possible to provide the valve housing with an extremely compact design, with all of the essential control and connection conduits being realised in the directional control valve spool or in the piston of the individual pressure compensator, while pump port, tank port, control port, etc. are provided in the valve housing. The latter may thus be used in a multiplicity of various valve assemblies essentially without modifications, whereas it is possible to carry out the individual adaptations with comparative ease by variation of the directional control valve spool and of the pressure compensator piston.
It is particularly advantageous if the end surfaces of the directional control valve spool are acted on by the control pressure which may, for example, be the individual load pressure of the user, a pressure derived therefrom, for example an artificially increased pressure, or the highest system load pressure, whereby it is ensured that an identical control pressure prevails on both control sides of the directional control valve.
A separate control conduit for actuating the individual pressure compensator piston may be omitted in that the control pressure present at the control sides of the directional control valve spool is guided through a control passage to the spring side of the pressure compensator piston.
For the case that the individual load pressure at the respective user is higher than the control pressure present, it is possible to design the pressure compensator piston to include a connecting bore through which the pressure compensator piston spring chamber may be connected to the piston front side upon a predetermined axial displacement of the pressure compensator piston, so that the individual load pressure is also present at the back side (spring chamber) of the pressure compensator piston. As a result of the above described control passage in the main spool, this individual load pressure is also passed on to the control sides of the directional control valve spool, where it is ensured that the control pressure corresponds to a respective highest system load pressure. In this case, the pressure compensator piston also fulfils the function of a shuttle valve as used in conventional solutions for passing on the highest load pressure in the system.
Axial displacement in the closing direction may most conveniently be limited by providing the pressure compensator piston with a radial collar which may be taken into contact with a correspondingly shaped shoulder of the spool bore. This shoulder may at the same time be employed for controlling open the connecting bore, so that the shoulder acquires a double function.
A valve assembly having particularly variegated applicability includes two directional control valve spools, each of which is designed as a hollow spool with an individual pressure compensator piston guided therein, so that in the valve housing merely the work ports, the tank port, the work port, the corresponding passages and the connecting passages for application of the control pressure to the front sides of the hollow spool must be realised.
The supply conduit to the user may be cut off leak-free through provision of an electrically releasable non-return cartridge valve, with only the screw-in portion for the non-return cartridge valve having to be provided in the valve housing.